1. Field of Invention
This application relates generally to electronic assemblies and more particularly to printed circuit boards used in electronic assemblies.
2. Discussion of Related Art
Electronic systems are often manufactured using multiple printed circuit boards called “daughter cards.” Each daughter card contains electronic circuitry. To assemble the system, the daughter cards are installed in a card cage or other mechanical support structure. Often, electrical connectors are mounted on the daughter cards to allow the circuitry on each daughter card to connect to circuitry in other parts of the electronic system. An example of a connector that is used on daughter cards is shown in U.S. Pat. No. 6,554,647 assigned to Amphenol Corporation. The assignee of that patent sells a connector under the name of GbX®.
In a backplane-daughter card configuration, the electronic system includes a printed circuit board called a “backplane.” The backplane has connectors on it that mate with the connectors on each of the daughter cards. Conductive paths within the backplane interconnect the connectors on the backplane so that electrical signals may be routed from one daughter card to another.
In a complex electronic system, many signals may be interconnected through the backplane. Signal paths within printed circuit boards are created on conductive traces that are laid out in layers. Backplanes with more connections than can fit on one layer have multiple layers of conductive traces. It is not unusual for a backplane to have 10 layers or more, called “routing layers,” containing signal traces.
It is desirable for a printed circuit board to have as few layers as possible. Boards with more layers cost more to manufacture. Having more layers can also increase the thickness of the board, which may undesirably increase the size of the system. Each layer of signal traces is separated from the next layer of signal traces by a ground plane to reduce interference between signals and all layers are separated by insulative material to avoid shorting the conductors on adjacent layers. Accordingly, adding one layer of signal traces increases the thickness of the board by more than the thickness of the signal traces.
Further, having a thicker board with interconnections between layers degrades the integrity of signals transmitted on the board. To make a connection between a connector mounted to a board and a signal trace within the board, a hole is drilled through the board and plated with metal to make a “via” to the signal trace. The connector is attached to the via near the surface of the board. The area where the attachment is made is called the “signal launch.” Various forms of signal launches are known. For example, a pad electronically connected to the plating in the via may be used as a signal launch for surface mount connectors. Other connectors have press fit contact tails. Press fit contact tails have compliant portions that may be forced into the vias. Regardless of the specific construction of the signal launch, the amount of distortion introduced by a via is often related to the thickness of the board.
In some circumstances, it is possible to limit the thickness of a printed circuit board by making the signal traces thinner or closer together so that fewer routing layers are required. There are, however, practical limits to the size and spacing of signal traces. If the traces are too close together, signals on one trace may interfere with signals on another trace, creating a form of distortion called “cross talk.”
In addition, narrow traces can increase the risk that the circuit board will be manufactured with a defect that shorts two traces together or creates an open circuit. Additionally, as the trace width decreases, the trace becomes more lossy, which can further decrease the integrity of signals transmitted through the backplane. While the negative effects of narrow traces can, to some extent, be overcome, techniques to avoid these problems add manufacturing cost or complexity. For example, tighter controls in the manufacturing process can be used to reduce the chance of defects. The board may also be made with an insulative material that is less lossy than the FR4 or similar material traditionally used to make printed circuit boards. However, special manufacturing materials or processes add cost.
The need to reduce the number of routing layers is particularly important in electronic systems such as routers and switches in which many signals are passed between daughter cards. Thick backplanes are often needed to route the required number of signals.
A particularly challenging configuration is called the “star routed” configuration. In the star routed configuration, a plurality of I/O cards are connected to cards in the center of the backplane, called “switch cards.” A particularly challenging routing configuration occurs in systems that use a redundant switch. The redundant switch contains I/O cards that each interface to several lines from outside the switch. Each of the I/O cards is connected to a switch card. The switch card can connect any signal from any I/O card to any I/O card so that signals may pass from any input to the switch to any output of the switch. The switch card and the I/O card are positioned as daughter cards. The connections between these cards is provided through a backplane.
The need to connect so many signals to the switch card requires many routing layers in the backplane. In a redundant switch, the number of signals that must pass through the backplane is even greater. A redundant switch includes a second switch card. Signals from each I/O card are also connected to the second switch card, which can perform all the operations of the first switch card, if the first switch card fails. Accordingly, the number of signals that must pass through the backplane of a redundant switch is double that of an ordinary switch.
It would be desirable to construct a backplane requiring a small number of routing layers for a redundant switch or other electronic systems with a large number of interconnections.